1. Field of the Invention
This invention relates to an engine-driven heat pump apparatus, for heating or cooling the air in a room, comprising an inside heat-exchanger installed in the room and an outside heat-exchanger installed outside the room, and, in particular, to such an apparatus with the use of exhaust heat from the engine to compensate for insufficient heat of evaporation. In addition, this invention relates to a method for stable operation of an engine-driven heat pump, in which exhaust heat from the engine is used to compensate for insufficient heat of evaporation, thereby stabilizing operation of the heat pump, especially in the heating mode during cold weather.
2. Background of the Art
A heat pump apparatus functions as a heater and a cooler by switching the flow of the refrigerant. That is, an inside heat-exchanger functions as a condenser for heating the room while it functions as an evaporator for cooling the room. An outside heat-exchanger functions in the opposite way. A problem in operating the heat pump apparatus is an insufficient heating power (or heating capacity) during cold weather. This problem will be explained with reference to a basic cycle of an engine-driven heat pump apparatus depicted and a p-i chart (pressure and enthalpy chart).
FIG. 8 shows a basic cycle of an engine-driven heat pump apparatus in the heating mode, and FIG. 9 shows a p-i chart of the basic cycle of the engine-driven heat pump apparatus.
When a compressor 2 is driven by an engine 1, a vaporized refrigerant in a state (pressure P.sub.1 and enthalpy i.sub.1) marked (1) in FIG. 9 is compressed in the compressor 2 and changed to a state (pressure P.sub.2 and enthalpy i.sub.2) marked (2) in FIG. 9, in which the refrigerant is under a high pressure with a high temperature. The power of the compressor 2 necessary to cause the change per unit weight of the refrigerant (the quantity of heat for compression), AL, is expressed as (i.sub.2 -i.sub.1).
The refrigerant under a high pressure with a high temperature is introduced to an inside heat-exchanger 39 functioning as a condenser, and liquefied therein as a result of radiating heat of condensation Q.sub.2 to the air in a room. The liquefied refrigerant, after passing through the inside heat-exchanger 39, is in a state (pressure P.sub.2 and enthalpy i.sub.3) marked (3) in FIG. 9, in which the refrigerant is sub-cooled as a result of radiated heat Q.sub.2 (i.e., i.sub.2 -i.sub.3) which heats the interior of the room.
The liquefied refrigerant in a state marked (3) subsequently undergoes reduction of pressure due to an expansion valve 40, and is changed to a state (pressure P.sub.1 and enthalpy i.sub.3) marked (4) in FIG. 9, in which a portion of the refrigerant is vaporized. The partially vaporized refrigerant is then introduced to an outside heat-exchanger 42 functioning as an evaporator.
Meanwhile, a cooling water, which circulates in a cooling water line via a water pump 35, absorbs exhaust heat from the engine 1 through an exhaust gas heat-exchanger 27 and the engine 1 itself, and exerts the absorbed heat on the refrigerant at the outside heat-exchanger 42. Thus, the refrigerant receives heat from both the outside air and the cooling water at the outside heat-exchanger 42, and vaporizes, in which process the refrigerant is superheated and returns to a state (pressure P.sub.1 and enthalpy i.sub.1) marked (1) in FIG. 9. After this the same operation as above is repeated. In the above, the quantity of heat Q.sub.1 the refrigerant receives at the outside heat-exchanger 42 is expressed as (i.sub.1 -i.sub.3).
In the above cycle, by exerting exhaust heat from the engine 1 on the refrigerant, the temperature in the heat cycle by the refrigerant is increased, thereby improving heating power (i.e., radiated heat Q.sub.2). Incidentally, a warm water, which has been heated by exhaust heat from the engine at an exhaust gas heat-exchanger, can be introduced into a warm water line passing through the liquid refrigerant inside an accumulator disposed upstream of the compressor (see Japanese patent laid-open Hei 5-180529/1993).
In the above, the lower the temperature in the room the higher the heating power required; however, when the temperature in the room is low, the temperature of the outside air is normally low, and when the temperature of the outside air is too low, the refrigerant passing through the outside heat-exchanger functioning as an evaporator cannot absorb sufficient heat from the outside air, i.e., short of heat of evaporation, thereby abating heating power. Thus, when the temperature of the outside air is low, a fan for enhancing heat-exchange between the cooling water and the refrigerant is stopped, and heat of evaporation is compensated for by exhaust heat from the engine through the cooling water. However, the exhaust heat from the engine is not sufficient when the temperature of the air inside or outside the room is too low.